The invention relates to a method for laser machining transparent materials, in particular for producing precise, localised changes of material inside a transparent body.
Generating very tine effects during laser material machining requires localised deposition of very small amounts of energy. When energy is deposited by linear absorption (single-photon absorption), the desired high precision of the material machining requires a small optical penetration depth of the laser radiation as well as a sufficiently short length of the laser pulse so as to avoid heat diffusion during the duration of the laser pulse.
In transparent materials such as glass, quartz, water, body tissue without pigments, or cells, energy can be deposited in a localised fashion only through non-linear absorption, that is to say by multiple-photon processes in the form of multi-photon ionisation and avalanche ionisation that lead to the formation of a plasma (quasi-free charge carriers in the material consisting of a mixture of electrons and ions). Since the occurrence of multi-photon processes is a non-linear function of the laser light intensity, this is referred to as “non-linear absorption”. And since the plasma formation rate above a threshold that depends on the material and laser parameters increases extremely strongly, in this parameter range the plasma formation process is also called “optical breakdown”.
A high degree of precision during machining material by non-linear absorption requires that spatially localised reproducible small amounts of energy can be introduced into the material (deposited). Good spatial localisation is above all achieved by focusing the laser pulses by means of aberration-free optics having a high numerical aperture.
In the prior art it is assumed that a much better way to introduce little energy in conjunction with a high degree of reproducibility can be by ultra-short laser pulses having a duration in the range of a few femtoseconds up to a few picoseconds rather than with longer pulses (nanoseconds or even microseconds). For this there are above all three reasons:                1. The threshold that can be detected experimentally for the optical breakdown is usually equated with the observation of plasma luminescence—above all in the case of pulse durations in the nanosecond range.        2. The energy threshold for the formation of luminescent plasmas decreases strongly with a decrease in the pulse duration—approximately by a factor of 200 when the pulse duration is reduced from 10 ns to 100 fs. For nanosecond pulses, occurrence of a luminescent plasma is therefore linked to a much higher energy density and therefore associated with much stronger mechanical laser effects and side effects of the actual ablation of material.        3. Close to the threshold for the optical breakdown the laser effects generated by the conventional nanosecond pulses have a much wider scattering range than the femtosecond effects.        
This has led to the view that the optical breakdown with nanosecond pulses be of a generally “statistic nature” while the femtosecond breakdown be “deterministic” and therefore better suited for reproducible machining of materials. The statement is furthermore justified that the occurrence of plasma luminescence during laser treatment with nanosecond pulses in principle conflicts with a precise localisation of the laser effects.
Regrettably, pulsed laser systems with pulse durations of less than 100 ps are complex systems with correspondingly high acquisition costs. Over and again compromises were sought that link the desire substantiated above for short-time pulses .with affordable equipment.
This is for example the case in the work by Colombelli et al., “Ultraviolet diffraction limited nanosurgery of live biological tissues”, Rev. Sci. Instrum., Vol. 75, 472-478 (2004), where a UV chip laser (triple-frequency Nd:YAG) with a pulse duration of 500 ps is successfully used as a laser scalpel among others for individual cells. Such microchip lasers are relatively low priced, but typically also work with nanosecond pulses and in principle cannot fall below pulse durations of a few 100 ps.
According to the teachings of DE 198 55 623 C1 precise machining of materials with nanosecond pulses in transparent media is possible for carrying out the known inside laser engraving of glass; here even pulse durations of 100 ns are used. The patent specification however expressly states that wavelengths outside the plateau region of the transmittancy should be used, that are those for which the material precisely does not have an optimum degree of transparency. The importance of the occurrence of linear absorption that is necessitated thereby is emphasised by the authors but not explained in more detail.
A method for detecting very small transient changes in material are suggested in another patent application of the applicant (DE 10 2007 003 600.2). It's about a method for laser perforation of cell membranes that is as gentle as possible, where the pulsed laser light generates bubbles in the immediate vicinity of the cells in the focus. The size determination of the bubbles can take place via measuring the oscillation time by detecting the behaviour over time of the change in light intensity of a probe laser beam (cw laser, preferably another wavelength) that is guided together with the pulsed radiation (fs to ps pulses) through the laser focus during machining. Bubble sizes of as low as 150 nm have been detected in this way.